The decision to specify energy-efficient windows in your new homes or remodeling projects is a relative no-brainer; in fact, they’ve long since passed mainstream status given energy codes, local requirements, and buyers’ awareness and demand for more efficient products. Energy Star-rated windows, which include low-E glazings and meet a certain energy-efficiency threshold, now make up more than half of the window market, according to the Department of Energy.

But builders know they often have to go even beyond Energy Star to qualify their homes for national and regional green building certification programs, and to meet buyer demand. The options don’t stop with Energy Star or even low-E and argon gas. The range of choices for energy-efficient glazings continues to grow with an ever-widening assortment of available efficiency levels and ratings. Here’s a run-through of energy-efficient glazing options and how to determine what’s right for your project and climate.

Minimum Requirements
Use of low-E window glazing, a microscopically thin, metallic window coating that reduces the amount of heat radiating through the surface, is now at about 56% in the residential window market, according to a study by the American Architectural Manufacturers Association and the Window and Door Manufacturers Association.

Even in areas with moderate climates, the minimum window you should install is a low-E Energy Star-rated product, which, beyond simply helping your homeowners save on energy costs, is required if you want your homes to qualify for federal tax credits, for many utility incentives, and for the Energy Star Homes or other green building program recognitions. In some cases, Energy Star-rated units are needed to meet local building codes.

To meet Energy Star requirements, products must be rated, certified, and labeled for both U-factor and Solar Heat Gain Coefficient (SHGC) by the National Fenestration Rating Council, and meet Energy Star criteria in one of four U.S. climate zones, which are divided based on the amount of heating and cooling typically required.

The lower the U-factor, the greater a window’s resistance to heat flow and the better its insulating value. Energy Star recommends a U-factor lower than 0.35 in northern climates, 0.40 in central climates, and 0.65 in southern climates.

The lower a window’s SHGC, the less solar heat it transmits. Energy Star recommends an SHGC of 0.55 or less in north/central climates and 0.40 or less in south/central and southern climates.

According to projections in “Residential Windows,” run on RESFEN software created by the Lawrence Berkeley National Laboratory, windows with the Energy Star-required U-factor of 0.35 and an SHGC of 0.44 could reduce annual energy costs of a typical 2,000-square-foot Minneapolis dwelling from around $1,350 to less than $1,100. Windows with an SHGC lower than the required 0.40 in Phoenix could drop annual energy costs from $900 to $700.

While higher efficiency windows usually cost more than basic units, the added efficiency provides immediate energy savings, says Ken Fonorow, president of Florida H.E.R.O. and an independent energy analyst and building science consultant. An additional $2,000 spent on more efficient windows equates to about $12 per month in the mortgage payment, but the homeowner could save $15 per month in energy costs, Fonorow says.

Next Generation
Window and glass manufacturers are providing glazing options with efficiency ratings that go well beyond Energy Star standards and have the potential to save homeowners even more on energy costs. The latest generation of window glass, such as Cardinal’s LoE3-366 and PPG’s Solarban 70XL, feature three low-E coatings (also known as “triple silver”) to further improve a window’s solar-heat-gain-blocking properties. Solarban, for example, has an SHGC of 0.27, compared with 0.39 for double silver and 0.75 for clear glass.

To lower the U-factor, window manufacturers are using additional films or panes of glass to create multiple insulating chambers, and they’re filling those cavities with argon or krypton gases, which are heavier and denser than air, so they slow down the convection currents within the window.

A triple-glazed window with an insulated vinyl or fiberglass frame, two low-E coatings, and two ¼-inch cavities filled with krypton gas (or ½-inch cavities filled with argon gas) can achieve U-factors as low as 0.18, or 0.13 in the center of the glass, which is 88% lower than a clear, single-pane unit, according to “Residential Windows.” However, switching to a triple-glazed glass package from a clear insulated glass unit could add as much as 30% or more to a window’s cost.

Installing such a unit with a standard vinyl or wood frame, with moderate-solar-gain low-E coatings on the glazings, for a U-factor of 0.28, would bring annual energy costs down further to just over $1,000 for the sample house in Minneapolis. Using the same system with low-solar-gain low-E coatings, creating an SHGC of 0.25, would bring energy costs down to just over $600 for the Phoenix home. Many green building programs recognize these benefits. For example, Denver’s Built Green Colorado program, which designates green homes by awarding points for achieving certain items on a checklist, awards four points for employing windows with the Energy Star-required U-factor of 0.35, but eight points for windows with a U-factor of 0.30. The Austin Energy Green Building Program awards three points for meeting Energy Star and two more points if the windows have an SHGC of 0.30 or lower.

If you haven’t already made the leap to more energy-efficient windows, Energy Star could provide a nudge as the program reevaluates its criteria this year. In an October 2007 letter to stakeholders, Energy Star program manager Richard Karney said the DOE would consider lowering the required U-factor in northern zones and SHGC in southern zones to between 0.20 and 0.30. The program also could require minimum visible light transmittance levels, among other adjustments, with the new criteria taking effect in 2009 at the earliest.

The impact of the new requirements will depend on where the final numbers end up, predicts John Meade, director of new business development for Southwall, which manufactures low-E Heat Mirror film. If the U-factor requirement is at the high end, around 0.29, manufacturers may be able to adjust their broad product line by insulating the window frames or changing the gas mix. But if the requirement is 0.25 or lower, manufacturers might have to create new Energy Star product lines that are triple-glazed, but also more expensive.

In the meantime, new, promising technologies continue to emerge. Sage Electrochromics, for example, offers electronically tintable glass that can change from dark to clear with the flick of a switch. Already an option on Velux skylights, the technology is still expensive, but could drop in price significantly with higher sales. Like low-E, tintable glass could change the way we look at-and through-windows.

Jeffrey Lee is associate editor for Building Products magazine.

Things to Consider
Here are some points to consider when considering super-efficient window glazings.

• Cost. Pros agree that the biggest deterrent is the initial cost. Keep an eye out for tax credits, and check with your local utility for incentives.
• Placement. The number and the orientation of the windows are key. In cold climates, windows should be oriented predominantly to the south to take advantage of passive solar gain. In southern climates, most windows should face north, where there is less solar exposure, or south, where they can be easily shaded with overhangs.
• Visibility. While tinting and shades reduce light, low-E coatings can protect against solar heat gain without losing as much visible light. Position windows with higher visible light transmittance in shaded areas, such as under porches.
• Durability and installation. All the other factors don’t mean a thing if the windows aren’t durable or aren’t installed properly.

Beyond Glass
When it comes to choosing an energy-efficient window, the glass isn’t the only thing that matters. Frame materials govern the physical properties of the windows, such as appearance, weight, and durability; have a large impact on the thermal performance; and affect the amount of maintenance the windows will require. U-factors for frame materials are usually not reported by manufacturers and depend on dimensions and design details. Choose a material that best fits your client’s needs and environmental goals.

• Aluminum. With a U-factor of 0.80 to 2.4, aluminum frames have high thermal conductance, resulting in poor overall window U-factors. “Thermal breaks” that slow the heat flow through the window can somewhat mitigate the disadvantage. Nevertheless, aluminum is durable and low maintenance, and has a high strength-to-weight ratio.
• Fiberglass. When filled with insulation, fiberglass window frames can have superior thermal performance similar to insulated vinyl, with U-factors as low as 0.20. However, they typically are more expensive than vinyl and aluminum, falling in the same price range as clad-wood and composite.
• Vinyl. Like wood, vinyl frames have good thermal performance, which can be improved with insulation and other design features for a U-factor between 0.20 and 0.60. They require very little maintenance, but the USGBC is studying the environmental concerns associated with the production and disposal of PVC.
• Wood. With a U-factor of 0.30 to 0.60, wood frames have better thermal performance than aluminum. Wood is also a renewable resource, and Forest Stewardship Council-certified windows are available. However, wood requires exterior paint and maintenance unless it is clad with vinyl or aluminum.
• Wood composites. Similar to composite decking, composite window frames are made from a combination of wood fiber and polymers for better moisture and decay resistance. They have similar or better thermal performance to wood, and can use wood and PVC scrap that might otherwise go to waste.

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